Isospectral twirling and quantum chaos

TL;DR

This paper introduces a unified framework using isospectral twirling to analyze quantum chaos measures, revealing how spectral properties and eigenvector choices influence chaos indicators and their asymptotic behavior.

Contribution

It establishes isospectral twirling as a comprehensive tool for understanding quantum chaos, connecting spectral and eigenvector effects, and demonstrating differences between chaotic and integrable systems.

Findings

Isospectral twirling unifies quantum chaos measures as expectation values.

Chaotic spectra (GUE) show distinct finite-time profiles from integrable spectra.

Asymptotic values depend on eigenvector properties, not just spectra.

Abstract

We show that the most important measures of quantum chaos like frame potentials, scrambling, Loschmidt echo, and out-of-time-order correlators (OTOCs) can be described by the unified framework of the isospectral twirling, namely the Haar average of a $k$-fold unitary channel. We show that such measures can then be always cast in the form of an expectation value of the isospectral twirling. In literature, quantum chaos is investigated sometimes through the spectrum and some other times through the eigenvectors of the Hamiltonian generating the dynamics. We show that, by exploiting random matrix theory, these measures of quantum chaos clearly distinguish the finite time profiles of probes to quantum chaos corresponding to chaotic spectra given by the Gaussian Unitary Ensemble (GUE) from the integrable spectra given by Poisson distribution and the Gaussian Diagonal Ensemble (GDE). On the other hand, we show that the asymptotic values do depend on the eigenvectors of the Hamiltonian. We see that the isospectral twirling of Hamiltonians with eigenvectors stabilizer states does not possess chaotic features, unlike those Hamiltonians whose eigenvectors are taken from the Haar measure. As an example, OTOCs obtained with Clifford resources decay to higher values compared with universal resources. Finally, we show a crossover in the OTOC behavior between a class of integrable models and quantum chaos.